1. Field of the Invention
The present invention relates to a bandsaw for cutting off slices from a workpiece, comprising a sawband which rotates over a system of pulleys and has a band back and a cutting edge. This cutting edge is situated opposite to the band back and is provided with a cutting surface. There is also a feed system for providing a relative movement between a workpiece and the cutting edge. Thus, as the sawband penetrates into the workpiece, a cutting gap is formed.
The present invention also relates to a process for cutting off slices from a workpiece, and in particular for cutting off semiconductor wafers, using this bandsaw.
2. The Prior Art
Rods or blocks of semiconductor material, such as for example silicon, are sliced into wafer-like units on an industrial scale. These units are predominantly needed as base material for the production of electronic components. Annular saws, frame saws and wire saws are usually used for cutting off slices from such workpieces. Bandsaws are also used, but only for dividing up large cast ingots by individual cuts, and for cutting off endpieces of crystals. Annular saws and wire saws are described in the chapter entitled "Silicon" in Vol. A23 of Ullmann's Encyclopedia of Industrial Chemistry, VCH Verlagsgesellschaft Weinheim 1993, on pages 734, 735, 739 and 740. A description of bandsawing can be found, for example, in Dubbel, Taschenbuch fur den Maschinenbau [Handbook for Machine Construction], Springer-Verlag Berlin, 18th Edition, pages T87 and T88.
Wire saws are currently used predominantly for the production of semiconductor wafers with relatively large diameters of 200 and 300 mm. This is because the waste when using wire saws is lower than the waste created when using annular saws. On the other hand, the use of annular saws has the advantage over the use of wire saws that the end of the workpiece can be machined before the cutting operation. Consequently, the semiconductor wafer produced after the cutting operation has a reference plane for further material-abrading machining steps. This machining of the end of the workpiece is described, for example, in DE-3613132 A1. Since bandsawing is a method in which semiconductor wafers can be produced sequentially, this is similar to annular sawing. Thus, the advantage of machining the end of the workpiece can also be utilized in conjunction with bandsawing. Because of the high cutting forces which arise when cutting off semiconductor wafers using a bandsaw, it has hitherto been necessary to use sawbands which have a relatively large cross-section. However, these sawbands cause a high level of waste, making it uneconomical to slice off semiconductor wafers using bandsaws. Very thin sawbands, which could be used to reduce the waste, have only a low degree of strength. This low strength is a disadvantage for producing precise cutting at a high feed rate, which is economically necessary.